Dynamic on chip slew rate control for mos integrated drivers

ABSTRACT

A slew rate control system for a circuit includes a temperature sensitive device generates a temperature signal. A temperature dependent current source generates a first current source signal. A slew rate controller activates in response to the temperature signal when the temperature exceeds a first threshold. The slew rate controller then generates a slew rate control signal by adjusting slew rate as a function of the temperature signal. The slew rate controller then activates the temperature dependent current source in response to the slew rate control signal.

TECHNICAL FIELD

[0001] The present invention relates generally to slew rate control, andmore particularly to dynamic on chip slew rate control.

BACKGROUND ART

[0002] Slew rate is a ratio of the rise or fall in voltage to the lengthof time required for that rise or fall. Consequently, slew rate tends tobe a controlling factor in the performance characteristics of electronicdevices. A device having a low slew rate can degrade the performance andspeed of a system containing the device, while a device having a highslew rate may not allow a system to react to changes in the state of thedevice and thereby cause errors in the system.

[0003] The effects of slew rates have led manufacturers of systemsrequiring processors, such as automotive electronic systems, to specifyperformance parameters into which device slew rate characteristics mustfall in order to properly operate within those systems. Processvariations between different fabrication methods, however, tend to causelarge variations in device characteristics, such as drain to sourcecurrent. Characteristic variations substantially bar devices fromconsistently exhibiting slew rate characteristics within the performancerequirements. Therefore, substantial need exists for a buffer thatallows adjustment of slew rate characteristics after a circuit or devicehas been manufactured.

[0004] The output current of typical semiconductor drivers varies withprocess, temperature, and supply voltage. To guarantee the desiredoperation for nominal values, typical drivers are designed to outputnominal currents, which are greater than minimum current required undernormal operating conditions. During rapid electrical processes, thesecurrents can potentially become several times greater than the minimumrequired. Large currents tend to result in high voltage slew rates,which inject noise into analog portions of mixed signal systems.

[0005] Power transistors, such as Metal-Oxide Semiconductor (MOS)transistors, dissipate large amounts of power in their collector-basejunctions. The dissipated power is converted into heat, which raises thejunction temperature. The junction temperature should not exceed acomponent specific maximum temperature, or the transistor will sufferpermanent damage. The range for typical transistors is between 150° C.and 200° C.

[0006] Most automotive power integrated circuits (ICs) have a slewcontrol on the output voltage to minimize radiated emissions incompliance with requirements in automotive electronics systems, whichtypically require slew rates around 1 V/μs. A difficulty encountered inmany circuits having very large inductive loads, however, is powerdissipation. For clamped inductive loads, a lower slew rate value causesan increase in switching loses and leads to a rapid increase in junctiontemperature.

[0007] Most Automotive ICs include thermal shutdown protection circuits,which are typically set at 10 to 25 degrees above the recommendedmaximum temperature of 150° C. up to which junctions are substantiallyoperable. After junction temperature reaches this set value, the outputdriver is turned off. This operation allows the device to protect itselfin case of thermal overload.

[0008] For reliability, it is desirable to prevent junction temperaturesfrom exceeding 150° C. This limitation is generally accomplished throughappropriate heat sinking and driver size. Under certain circumstances,however, these solutions are cost prohibitive and impractical. Duringdevelopment or early production phases, thermal issues require manualadjustments to the slew rate of the output driver. Developers anddesigners resultantly trade off between improved emissions at loweroperating temperatures and improved thermal performance at higheroperating temperatures. A system for controlling slew rate at varioustemperature conditions would substantially solve this problem.

[0009] The disadvantages associated with current slew rate controltechniques have made it apparent that a new technique to regulate slewrate is necessary. The new technique should reduce power dissipationgenerated from switching loses in integrated circuits with large outputdevices at substantially low cost and should also enable circuitdesigners to fine-tune the output voltage slew rate as a function oftemperature to obtain an optimal overall thermal performance. Thepresent invention is directed to these ends.

SUMMARY OF THE INVENTION

[0010] The present invention provides a system and method for slew ratecontrol. The present invention also provides a method for dynamic, onchip control of slew rate.

[0011] In accordance with one aspect of the present invention, a slewrate control system for a circuit, which includes a temperaturesensitive device adapted to generate a temperature signal, is disclosed.A temperature dependent current source is adapted to generate a firstcurrent source signal. A slew rate controller is adapted activate inresponse to the temperature signal when the temperature exceeds a firstthreshold. The slew rate controller is further adapted to generate aslew rate control signal by adjusting slew rate as a function of thetemperature signal. The slew rate controller is further adapted toactivate the temperature dependent current source in response to theslew rate control signal.

[0012] In accordance with another aspect of the present invention, amethod for reducing power dissipation in a circuit having an outputdevice, comprising: activating a slew rate controller in response to atemperature rise in an element of the circuit above a threshold;regulating said slew rate as a function of said temperature increaseabove said threshold; and generating a slew rate control signal, isdisclosed.

[0013] One advantage of the present invention is that it facilitatessubstantially optimal performance at lower temperatures and optimalthermal performance at higher temperatures. A further advantage is thatit protects the circuit during thermal overload. Additional advantagesand features of the present invention will become apparent from thedescription that follows and may be realized by the instrumentalitiesand combinations particularly pointed out in the appended claims, takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a slew rate control system in accordance with anembodiment of the present invention;

[0015]FIG. 2 is a schematic diagram of the slew rate control system ofFIG. 1;

[0016]FIG. 3 is a graph of current as a function of temperature for atransistor of FIG. 2;

[0017]FIG. 4 is a graph of the output voltage as a function oftemperature for the system of FIG. 2;

[0018]FIG. 5 is a graph of the output voltage as a function oftemperature for the system of FIG. 2; and

[0019]FIG. 6 is a block diagram of a method for reducing powerdissipation due to slew rate in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

[0020] The present invention is illustrated with respect to slew ratecontrol system, particularly suited to the automotive field. The presentinvention is, however, applicable to various other uses that may requireslew rate control, as will be understood by one skilled in the art.

[0021] Referring to FIG. 1, a slew rate control system 10, includes atemperature sensitive device 12 electrically coupled to a slew ratecontroller 14. The slew rate controller includes a temperature activatedcompensation current trip point 16 and a configurable linear slew ratecontrol 18. The slew rate controller is electrically coupled to atemperature dependent current source 20, which sends signals through again stage 22, and a temperature independent current source 24. Adder 28is electrically coupled to the aforementioned current sources 20, 24. Apush-pull current amplifier 30 is electrically coupled to the adder 28.A power transistor 32, electrically coupled to the push-pull currentamplifier 30, sends slew rate controlled signals to the inductive load34.

[0022] The temperature sensitive device 12 generates a temperaturesignal. The slew rate controller 14 activates in response to thetemperature signal when the temperature exceeds a threshold, i.e. whenthe temperature activated compensation current trip point controllercomponent 16 activates. The slew rate controller 14 further generates aslew rate control signal by adjusting the slew rate as a function of thetemperature signal. The aforementioned operation is conducted in theconfigurable linear slew rate control component 18, which also controlsthe maximum allowable temperature for circuit components. The slew ratecontroller 14 still further activates the temperature dependent currentsource 20 in response to the slew rate control signal.

[0023] The temperature dependent current source 20 generates a currentsource signal, which is amplified in a gain stage 22 to generate thefirst current source signal. The temperature independent current source24 generates a second current source signal. Adder 28 adds the firstcurrent source signal and the second current source signal and therebygenerates an adder signal. A push-pull current amplifier 30 amplifiescurrent of the adder signal and thereby generates a load signal. A powertransistor 32 receives the load signal and transfers the load signal tothe inductive load 34.

[0024] Referring to FIG. 2, a schematic diagram of the slew rate controlsystem 10 of FIG. 1 is illustrated. The present invention is illustratedwith respect to a plurality of transistors, diodes and resistors in acircuit 41. It will, however, be apparent to one skilled in the art thatthis serves only as an example of one embodiment, and numerous othercombinations of electronic circuit components, such as integratedcircuits, may alternately be used.

[0025] The system is illustrated with typical denotations used in thefield of electronics, such as “M” for MOSFETs (Metal Oxide SemiconductorField Effect Transistors), “Q” for BJTs (Bipolar Junction Transistors),“D” for diodes, “R” for resistors, “VCC” and “REF” for referencevoltages, “GND” for the common ground, and “V” for voltage.

[0026] The temperature sensitive device generates a temperature signal.In the present embodiment, the temperature sensitive device 12 includesat least one transistor (Q18 and Q19) and the temperature signal is theresultant response of saturation of the transistor Q18 due totemperature increase. The temperature sensitive device 12 is alternatelyembodied as a temperature sensor measuring or sensing increases intemperature in or around either a component, array of components,component junction, or a combination thereof, as will be understood byone skilled in the art.

[0027] The temperature dependent current source 20 includes Q16, Q18,Q19, Q20, Q24, R1, R7 and R8. Q18 is biased by a voltage divider 38 froma bandgab reference 40 generating a fixed voltage of 1.42V. The bandgabreference 40 includes Q1, Q2, Q4, Q5, Q6, Q7, Q8, Q15, R10, R11, R15,and R18, and is designed in the illustrated system 10 to maintain systemtolerance, as will be understood by one skilled in the art.

[0028] At elevated temperatures, the voltage of the base emitterjunction (VBE) of Q18 drops linearly as temperature increases. Once theVBE becomes lower than the bias voltage by a value high enough toforward bias the junction, Q18 begins conducting and Q19 resultantlyoperates in the active region. Q19 is biased by Q28, Q16, Q24, R1 andQ18.

[0029] Referring to FIGS. 2 and 3, the current generated by Q19 as afunction of temperature is illustrated. The current stays close to zerountil the temperature reaches approximately 80° C. and then increases astemperature increases. When the temperature reaches approximately 120°C., Q19 is forced into saturation, and the current stays relativelyconstant. The compensation current is amplified in the gain stage 22,which includes Q24 and Q25, to generate a first current source signal.The gain stage 22 magnifies the relatively small signal that is usuallyproduced from the temperature dependent current source 20. The firstcurrent source signal is added to the fixed current generated by Q11,and the resultant current or load signal is then directed to the powerMOSFET device M2 (power transistor 32) through M16 and M17.

[0030] The slew rate controller 14 includes a means for linearadjustment of slew rate. R5, R6, and Q18 are the temperature activatedtrip point, and R7, R8, and Q 19 are the configurable linear slew ratecontrol. The slew rate controller activates in response to thetemperature signal when the temperature exceeds a threshold which, forthe present invention, is approximately 120° C. (the saturationtemperature for Q19). The threshold is adjusted either through controldevices, known software control schemes, or, as illustrated, throughmanual adjustments varying the resistance of R6. R7 controls the slopeof the current and is also adjusted either through control devices,known software schemes or, as illustrated, through manual adjustmentsvarying the resistance of R7.

[0031] Referring to FIGS. 4 and 5, graphs of the output voltage as afunction of temperature for the system 10 of FIG. 1 for different R6 andR7 values, are illustrated. FIG. 4 illustrates modification of slew ratecompensation when the dynamic slew rate control system 10 is activatedat 125° C. FIG. 5 illustrates modification of slew rate compensationwhen the dynamic slew rate control system 10 is activated at 90° C.FIGS. 4 and 5 illustrate that slew rate is modified by large amounts atdifferent temperatures by varying R6 and R7.

[0032] Referring again to FIG. 2, the slew rate compensation system 10includes resistors with low thermal coefficients. For alternateembodiments, i.e. resistors with high thermal coefficients, a thermalcoefficient compensation device, such as a VBE multiplier, is includedin the system 10 and added in series to the resistors. Such a thermalcoefficient compensation device adds a fraction of negative thermalcoefficients to the positive thermal coefficients generated by theresistors and thereby compensates for resistors with high thermalcoefficients.

[0033] The slew rate controller 14 terminates the slew rate signal whenthe temperature reaches a maximum temperature allowable for at least oneelement of the circuit 41. R8 sets the final desirable value of thecompensation current and therefore controls worst-case performance whenthe circuit 41 is active. R8 is also controlled either through controldevices, software familiar in the art, or, as illustrated, throughmanual adjustments varying the resistance of R8.

[0034] A temperature independent current source 24, including Q3, Q10,Q11, Q12, Q13, Q14, Q22, Q23, R26, and D4, generates a second currentsource signal. Q11 generates the slew rate current. The current isindependent of temperature variations because of the zener reference(D4) and the adding Q13 and Q14 to compensate for the negative thermalcoefficients generated by the VBE of Q11 and Q10, as will be understoodby one skilled in the art. Therefore temperature variations have only aminimal effect on current mirrored by Q23 and Q22.

[0035] An adder 28 (including M16, M17, and M21) sums the first currentsource signal and the second current source signal and thereby generatesan adder signal.

[0036] A push-pull current amplifier 30 amplifies current of the addersignal and thereby generate a load signal. Push-pull transistors arebiased at zero current and conduct only when the signal is present. Thecircuit operates where M20 and M22 push (source) current into the load34 (L1, R2, and D6) when voltage coming into the push-pull currentamplifier 30 is positive and M23 and M24 pull (sink) current from theload when the incoming voltage is negative. When the pulse-widthmodulated (PWM) signal from the PWM source 49 is low, the gate of M2 ischarged through M22 and M20. When the PWM signal is high, M23 and M24are turned on while M20 and M22 are turned off, this discharges the gateof M2. Resultantly, the slew rate can be greatly modified at anytemperature by varying R6 and R7.

[0037] Referring to FIG. 6, a block diagram of a method for reducingpower dissipation generated from at least one switching operation in acircuit having an output device, in accordance with an embodiment of thepresent invention, is illustrated. Logic starts in inquiry block 50where an inquiry is made whether the temperature of a temperaturesensitive component is above a predetermined threshold. For a negativeresponse, the block diagram returns to the start function.

[0038] Otherwise, operation block 52 initiates, and the slew ratecontroller activates. The slew rate controller then adjusts the slewrate as a function of temperature, as was previously mentioned, andgenerates a slew rate control signal. In operation block 54, thetemperature dependent current source, which operates in response to theslew rate control signal, and the temperature independent current sourceare summed to generate a load signal.

[0039] For embodiments in which the load signal includes a weak voltagesignal, the voltage of the load signal is amplified in operation block56, as will be understood by one skilled in the art. The current of theload signal is amplified in operation block 58 through the push-pullcurrent amplifier.

[0040] A check is then made in inquiry block 60 whether the temperatureof the temperature sensitive component is above a maximum, which isideally set by the slew rate controller. Block 60 is illustrated assequential to block 58 however; this check is ideally made continuouslyduring the functioning of the aforementioned circuit. For a positiveresponse, operation block 62 activates and the output transfer unit(power transistor) is shut down. Important to note is that the slew ratecontroller alternately shuts down various other components orcombinations thereof to protect the circuit, as will be understood byone skilled in the art.

[0041] Otherwise operation block 64 activates, and the load signal istransferred to the inductive load through the power transistor.

[0042] In operation, the slew rate controller automatically increasesthe output slew rate after a junction temperature reaches a presetvalue. If the temperature continues to rise, the slew rate controllercontinues to increase the slew rate until it reaches the maximum slewrate controllable by the circuit elements. If the temperature continuesincreasing, thermal shutdown turns off the output device. The presentinvention thereby generates the optimal performance at lowertemperatures and optimal thermal performance at higher temperaturesthrough dynamic adjustments to the slew rate.

[0043] From the foregoing, it can be seen that there has been brought tothe art a new slew rate control system, which allows circuits toautomatically increase output slew rates after a junction temperaturereaches a preset value. It is to be understood that the precedingdescriptions of various embodiments are merely illustrative of some ofthe many specific embodiments that represent applications of theprinciples of the present invention. Numerous and other arrangementswould be evident to those skilled in the art without departing from thescope of the invention as defined by the following claims.

In the claims:
 1. A slew rate control system for a circuit, comprising:a temperature sensitive device adapted to generate a temperature signal;a temperature dependent current source adapted to generate a firstcurrent source signal; and a slew rate controller adapted activate inresponse to said temperature signal when said temperature exceeds afirst threshold, said slew rate controller further adapted to generate aslew rate control signal by adjusting a slew rate as a function of saidtemperature signal, said slew rate controller further adapted toactivate said temperature dependent current source in response to saidslew rate control signal.
 2. The system of claim 1 wherein saidtemperature sensitive device comprises a transistor.
 3. The system ofclaim 1 wherein said temperature sensitive device comprises atemperature sensor.
 4. The system of claim 1 wherein said slew ratecontroller terminates said slew rate signal when said temperaturereaches a maximum temperature value for at least one element of thecircuit.
 5. The system of claim 1 wherein said circuit further comprisesa temperature independent current source adapted to generate a secondcurrent source signal.
 6. The system of claim 5 further comprising anadder is adapted to sum said first current source signal and said secondcurrent source signal and thereby generate an adder signal.
 7. Thesystem of claim 5 further comprising a push-pull voltage amplifieradapted to amplify a current of said adder signal and thereby generate aload signal.
 8. The system of claim 7 further comprising a powertransistor adapted to receive said load signal and transfer said loadsignal to an inductive load.
 9. The system of claim 1 wherein thecircuit comprises a plurality of transistors.
 10. The system of claim 1wherein said controller comprises a means for manual adjustment of slewrate.
 11. The system of claim 1 wherein said controller comprises ameans for linear adjustment of slew rate.
 12. The system of claim 1further comprising a bandgab reference.
 13. A method for reducing powerdissipation in a circuit having an output device, comprising: activatinga slew rate controller in response to a temperature rise in an elementof the circuit above a threshold; regulating said slew rate as afunction of said temperature increase above said threshold; andgenerating a slew rate control signal.
 14. The method of claim 13wherein activating comprises sensing said temperature rise.
 15. Themethod of claim 13 wherein regulating comprises manually adjusting saidslew rate controller.
 16. The method of claim 13 further comprisingterminating said slew rate control signal when said temperature reachesa maximum temperature value for at least one element of the circuit. 17.A system for reducing power dissipation in a circuit adapted to supplyan output voltage to an inductive load, comprising: a temperaturesensitive device adapted to generate a temperature signal; a temperaturedependent current source adapted to generate a first current sourcesignal; a slew rate controller adapted activate in response to saidtemperature signal when said temperature exceeds a threshold, said slewrate controller further adapted to generate a slew rate control signalby adjusting a slew rate as a function of said temperature signal, saidslew rate controller further adapted to activate said temperaturedependent current source in response to said slew rate control signal; atemperature independent current source adapted to generate a secondcurrent source signal; an adder adapted to sum said first current sourcesignal and said second current source signal and thereby generate anadder signal; a push-pull voltage amplifier adapted to amplify currentof said adder signal and thereby generate a load signal; and a powertransistor adapted to receive said load signal and transfer said loadsignal to an inductive load.
 18. The system of claim 17 wherein saidtemperature sensitive device comprises a transistor.
 19. The system ofclaim 17 wherein said temperature sensitive device comprises atemperature sensor.
 20. The system of claim 17 wherein said slew ratecontroller terminates said slew rate signal when said temperaturereaches a maximum temperature value for at least one element of thecircuit.
 21. The system of claim 17 wherein said controller comprises ameans for manual adjustment of slew rate.
 22. The system of claim 17wherein said controller comprises a means linear adjustment of slewrate.
 23. The system of claim 17 further comprising a bandgab reference.